Evaluating FCC flue gas emission-control technologies
Comparing technologies for reducing FCCU flue gas emissions. Special emphasis is placed on selecting cost-effective solutions, particularly in the area of minimising SOx, NOx, CO and particulate emissions from operations in either full-burn or partial-burn mode
Martin Evans, Intercat (Johnson Matthey)
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So concerns grow over atmospheric pollution and its effects on the environment and human health, governmental agencies worldwide are adopting legislation regulating the emissions of a variety of pollutants from industrial sources. In the US, the Environmental Protection Agency (EPA) has pursued civil enforcement actions against many refiners regarding alleged violations of the Clean Air Act of 1990.
Today, 20 US refining companies representing 85% of US refining capacity have reached voluntary settlements, known as consent decrees, with the EPA. In these consent decrees, the settling refiners have agreed to pay civil penalties when applicable, but, more importantly, have agreed to perform significant environmental projects to reduce future pollutant emissions. As of October 2007, settling companies have agreed to pay civil penalties of $70 million, invest more than $4.8 billion in control technologies and perform supplemental environmental projects valued at approximately $65 million.1
The negotiation of a consent decree for a given company and refinery is a complex process that is in principle driven by the strength and severity of the Clean Air Act violations alleged against a company by the government and the company’s desire to avoid litigation. For the FCC unit, the EPA has target goals for SO2 emissions limits of 25 ppm, and for NOx emissions limits of 20 ppm. In addition, the government is targeting reductions in CO, particulate matter (PM), benzene and volatile organic compound (VOC) emissions.
In Europe, many EU countries already have local regulations aimed at reducing SOx, NOx, CO and PM emissions from industrial sources, including refineries. These pollutants are specifically targeted because of concerns about their deleterious effects on the environment and their impact on human respiratory health issues, particularly in those communities that surround the refineries. However, because these pollutants have cross-border impacts, the European Commission has also enacted directives to unify the control of pollutant emissions.
As of October 2007, the Integrated Pollution and Prevention Control (IPPC) directive places requirements on refineries to implement Best Available Techniques (BAT) to minimise emissions of SOx, NOx and PM into the environment. The European regulations do not propose specific emissions limits or prescribe the use of any specific technology. Rather, they allow member states to take into account the technical characteristics of the installation concerned, its geographical location and the local environmental conditions. Most member states are further delegating their obligations under the directive to regional or local authorities. Thus, emissions limits are being set locally and vary widely within Europe.2
Regulations in other regions vary country by country. In Asia, many countries also have regulations limiting SOx emissions from large industrial point-source emitters. These limits tend largely to be set by the local communities. In Canada, the regulatory limits on SOx and NOx emissions from a refinery vary and can be based on the concentration of the pollutants detected at monitoring stations in surrounding communities. For at least one Canadian refinery, SOx emissions from the refinery are being limited to meet an ambient SO2 detection limit of 0.190 ppm in the surrounding community.3 The 0.190 ppm limit is a level that has been determined by the World Health Organization (WHO) as being a scientific-based, health-protective guideline value for ten-minute peak exposures.
Canada is also leading the way in taking action on limiting greenhouse gas emissions from industrial sources. These gas emissions from a refinery may include carbon dioxide, water vapour, methane, nitrous oxide and ozone. The Canadian plan, entitled Turning the Corner: An Action Plan to Reduce Greenhouse Gases and Air Pollution, which was unveiled in April 2007, sets mandatory reduction targets for major industries that produce greenhouse gases. This plan calls for a 6% improvement each year from 2007–2010, giving an enforceable 18% reduction from the 2006 emissions level. The plan then requires a 2% annual improvement after 2010.4
Within an oil refinery, the FCC unit is typically the largest single emitter of airborne emissions. Significant quantities of regulated pollutants are emitted by the FCCU in the flue gas stream. These pollutants include SOx, NOx, CO, CO2 and PM (both total PM and micro-particulates — ie, PM10 and PM2.5). As a result, many refineries target the FCCU when they find that they must implement measures to reduce refinery emissions of these pollutants.
Controlling FCC SOx emissions
Sulphur present in the feedstock is the source of SOx emissions from the FCCU. Typically, 2–10% of feed sulphur ends up as sulphur contained in the coke on spent catalyst. When this coke is burned off the catalyst in the FCC regenerator, the sulphur is oxidised to SO2 and SO3 (SOx).
Unless removed, these SOx species will be emitted into the atmosphere via the FCC flue gas. To achieve the lower level of SOx emissions required by the regulatory directives, the refinery has several options. These options can be broadly classified into three categories:
- Feed selection and pre-treatment to reduce sulphur levels in the FCC feed
- Use of SOx-reduction additives to convert SOx as it is formed in the FCC regenerator to H2S in the reactor, where it is then routed to the sulphur-recovery unit (SRU) and ultimately converted into elemental sulphur
- Hardware solutions, which remove SOx from the FCC flue gas downstream of the FCCU before it is released to the atmosphere.
From these options, the selection of low-sulphur feeds is an option available to only a few refiners because of the limited availability of these feeds and their high cost. Hydrotreating of the FCC feed is a viable option for removing sulphur and improving the product yields in the FCCU. Unfortunately, the feed hydrotreater option requires a significant capital investment and ongoing high operating costs, and does not provide an adequate payout for many refiners.
Since the FCCU is typically responsible for emitting up to 90% of all SOx emissions from the refinery, most attention has been paid to implementing technologies that reduce SOx from the FCC flue gas stream. These technologies are listed in Table 1.
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